A cable network delivers services such as digital television, Internet, and Voice-over-IP (VoIP) phone service. A cable network has a controlling center or “head end”, which controls video and data traffic in the network by generating or distributing video and data signals. The signals are delivered over a tree-like network of a broadband coaxial cable termed “cable plant”. Digital television signals are broadcast from the headend to a trunk of the cable plant, and delivered to subscribers' homes connected to branches of the cable plant. In going from the headend to subscribers, the signals are split many times, and are attenuated in the process. Accordingly, a strong downstream broadcast signal is required, so that the signal level at the subscribers' premises is strong enough to be reliably detected. Upstream signals from the subscribers' homes carry phone and Internet traffic. The upstream signals propagate from the branches of the cable plant towards the headend of the network.
Upstream and downstream signals occupy separate frequency bands referred to as upstream and downstream frequency bands. Downstream information channel signals co-propagate in the downstream frequency band, and upstream signals co-propagate in the upstream frequency band. The frequency separation of the upstream and the downstream signals allows bidirectional amplification of these signals, which propagate in a common cable in opposite directions. In the United States, the upstream spectral band typically spans from 5 MHz to 42 MHz, while the downstream spectral band typically spans from 50 MHz to 860 MHz.
The upstream and downstream signals are prone to impairments and interference. Oxidized connectors may act as electrical diodes distorting the downstream signals by generating frequency harmonics, which may negatively impact both upstream and downstream signal paths. Aging equipment, such as signal boosters and amplifiers, may also distort the signals and add harmonics and “ringing” at unwanted frequencies. Another source of impairments is external electrical interference, termed “ingress noise”. Despite electrical shielding of the cable, outside signals may find their way into, and become guided by the cable. Shielding punctures, especially at customers' premises, improper installation, interference from closely placed high-current electrical equipment, etc., all contribute to accumulation of ingress noise. Furthermore, a cable plant may act as a receiving radio antenna. Thanks to its large size, a cable plant may pick up signals from otherwise unlikely sources, such as aviation radars.
The impairment situation worsens as new customers are added to an existing cable network. The cable plant is extended by adding more splitters and connectors, amplifiers, and long runs of coaxial cable to new locations. When a cable plant is expanded, a probability of downstream and upstream signal impairments increases. Accordingly, growth of extent and functionality of cable based networks must be matched by a growing effort to assure quality of existing services via periodic testing and maintenance of the networks.
Tracing a source of impairment is a common task in cable network maintenance and troubleshooting. To find an origin of noise, a technician travels from node to node, measuring noise levels in various branches of the cable plant. In practice, a technician decides on the origin of noise by taking a noise level measurement on a common leg of a signal amplifier/splitter/combiner, and comparing the measured noise level to noise levels on individual legs, which are connected to branches of the cable plant. Once a “faulty” branch is identified, the technician consults a cable plant map, finds a location corresponding to termination of the faulty branch, travels to that location, and repeats the measurement.
In situations where a noise source cannot be easily identified, a technician may be tempted to quickly disconnect a suspect branch from a cable plant, to see if the noise disappears. Such practice, although allowing the technician to find impairments quicker, is generally discouraged by technician's supervisor, because it interrupts all data and television services to many customers. With cable companies always trying to improve data transfer reliability, purposefully removing service should be avoided.
Another time-consuming problem of cable network interference troubleshooting is related to intermittent character of many ingress noise sources. Noise related to bursts of defective or old cable modems may come at quasi-random periods of time. Furthermore, noise related to a transmission of a particular upstream channel may or may not impact that channel, leading to puzzling situations where a detectable noise does not impact a particular channel, while upstream packet errors appear on an apparently noise-free channel, due to the noise being somehow synchronized to the packet transmission, or occurring so rarely that a measured frequency spectrum of a cable network signal does not show an appreciable level of noise.
Zinevich in US Patent Application Publication 2008/0320541 discloses a system for locating an impairment in a cable network including a plurality of “encoders” placed throughout a cable network. The function of the encoders is to uniquely modulate the noise floor at locations where the encoders are installed. An “impairment detector” is placed at a headend of the cable network. The impairment detector is configured to identify noise location(s) by analyzing noise modulation properties. While the system of Zinevich enables remote identification of noise sources in a cable network, it requires installation of many encoders throughout the cable network, which may be costly. Furthermore, intermittent noise, and/or noise impacting only certain transmission channels, is not always detectable with Zinevich system.
In view of the foregoing, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for testing cable networks.